1. Field of the Invention
The invention is directed to an electrolytic capacitor having an anode body that is surrounded by a housing. A cathode contact is located on an upper side of the anode body.
2. Description of the Related Art
The German patent document DE 198 46 936 C1 discloses capacitors of the initially cited species in which the anode body is a porous sintered body of tantalum powder. An anode contact projects from the inside of the anode body. A. cathode contact that is electrically conductively connected to the outside of the anode body is located on the upper side of the anode body. The anode body is extrusion-coated with a plastic housing.
The known tantalum electrolytic capacitors have the disadvantage that bubbles often form between the housing and the cathode contact during manufacture of the housing. The creation of these bubbles is explained by a sudden change in the coefficient of thermal expansion of the epoxy duroplastics usually employed for the extrusion coating in the range of the glass transition temperature of these duroplastics. This glass transition temperature is reached in the extrusion-coating of the anode body with the plastic or is even exceeded during this process. Due to the sudden, pronounced thermal expansion of the plastic mass, bubbles easily arise at the surface of the component where the thickness of the housing is relatively slight, i.e. the housing layer located over the cathode contact separates from it. Moisture can easily deposit in the cavity formed by the bubbles and this can lead to a sudden tearing of the housing due to sudden vaporization given thermal stressing of the capacitor. In addition, the housing can also easily tear at the location of the bubble given external mechanical stresses.
Another condition promotes the formation of bubbles. The plastics employed for the extrusion-coating of the anode body usually contain an adhesion-reducing agent in the form of waxes or in the form of phenols. This agent permits the plastic, following the extrusion-coating of the anode body, to easily detach from the injection molding tool in the form of an injection mould that is employed for the extrusion coating. Iron materials are usually employed for injection moulds. Accordingly, the adhesion reducing agents employed in the plastic mass are suitable for reducing the adhesion of the plastic on iron surfaces. Since, over and above this, the cathode contacts also usually contain iron, the adhesion of the housing is also reduced at the surface of the cathode contact.
Attempts have previously been made to solve the problem of bubble formation in that the wall thickness of the housing above the cathode contact constructed with a relatively great thickness. Due to its mechanical stability, a high wall thickness can prevent the formation of the bubble. This measure, however, has the disadvantage that it deteriorates the volume utilization of the capacitor, i.e., the capacitance available per volume.
Another solution to the problem that has been previously attempted is to provide a further process step in which the housing is hardened afterwards. This attempted solution has the disadvantage that an additional process step is needed that increases the manufacturing costs for the component.
An object of the present invention is to provide a capacitor of the species initially cited in which the risk of a lift-off of the housing part lying above the cathode contact is diminished.
This object is achieved by a capacitor and an appertaining method of manufacturing a capacitor, the capacitor comprising: a housing; an anode body that is surrounded by the housing; an anode contact projecting out of an inside of the anode body; an adhesion layer; and a cathode contact proceeding along an upper side of the anode body, the cathode contact having its housing side coated with the adhesion layer.
Advantageous developments of the invention include where the housing comprises an injection-moldable plastic, which may contain an agent that reduces adhesion of the plastic to ferriferous surfaces. The anode body may be extrusion-coated by the housing. The cathode contact may be formed of a ferriferous material of a system carrier. The cathode contact may be configured in a sheet metal strip shape. The adhesion layer may be galvanically applied on the cathode contact. The adhesion layer may be 0.1 through 0.5 xcexcm thick. The adhesion layer may comprise silver. And the housing over the cathode contact may be 2 through 5 mm thick.
A capacitor is provided that comprises an anode body. The anode body is surrounded by a housing. Furthermore, an anode contact is conducted out of the inside of the anode body, this anode contact projecting from the inside of the anode body. Additionally, the capacitor comprises a cathode contact proceeding along the upper side of the anode body, this cathode contact being coated with an adhesion layer.
By providing an adhesion layer at the housing side of the cathode contact, the adhesion of the housing to the cathode contact can be improved and the risk of bubble formation can also be reduced.
In particular, the housing can contain an injection-moldable plastic, this offering the advantage that the housing can be simply and cost-beneficially manufactured in great numbers using injection molding.
Furthermore, the injection-moldable plastic can contain an agent that reduces the adhesion of the plastic at iron-containing surfaces. Such an agent can, for example, be a wax or one or more phenols as well. This type of agent makes it possible to manufacture the housing of the capacitor with ferriferous injection moulds.
An anode body that is extrusion-coated by the housing is obtained by extrusion-coating the anode body with an injection-moldable plastic.
Advantageously, the cathode contact can be formed of the ferriferous material of a system carrier. System carriers are advantageously employed in order to manufacture a plurality of capacitors in great numbers in a simple and economical way. The basis is formed by a band-shaped system carrier to which a plurality of anode bodies is secured. After being detached, the anode bodies are extrusion-coated with the injection-moldable plastic. System carrier materials that are composed of iron or are at least ferriferous are usually employed. For example, a system carrier material could be composed of iron/nickel steel.
The cathode contact can advantageously comprise the shape of a sheet metal strip as can be acquired from a system carrier in standard processes. Such a sheet metal strip also has the advantage that a relatively large contact surface is available for the contacting of the cathode contact to the anode body of the capacitor.
For example, silver, copper, tin or even lead can be employed as a material for the adhesion layer. In general, all materials that improve the adhesion of the cathode contact to the housing are suitable. When using a plastic that contains an agent that reduces the adhesion of the plastic at ferriferous surfaces, particular care must be exercised to see that the adhesion layer covering the cathode contact contains very little or even no iron.
Experiments with system carriers already provided with an adhesion layer before the detaching have shown specifically that silver is especially well-suited as material for the adhesion layer. Silver has the advantage that, first, it does not melt that easily or volatilize in the relatively high temperatures occurring during operation of the capacitor and, thus, can also not emerge from the capacitor. Furthermore, its has been shown that silver has an adequate bendability characteristic or flexibility, which is required for bringing the capacitor formed of the system carrier into its final shape by bending the cathode contact.
Advantageously, the adhesion layer can be galvanically applied on the cathode contact. Such a galvanic process has the advantage that a plurality of cathode contacts or system carries can be simultaneously coated in a single electroplating bath.
Advantageously, the adhesion layer may be 0.1 through 0.5 xcexcm thick. A thinner adhesion layer might risk deterioration of the adhesion to the housing. Particularly when using silver for the adhesion layer, thicker adhesion layers have the disadvantage that high costs are incurred for their manufacture.
As a result of providing the adhesion layer at the housing side of the cathode contact, the thickness of the housing over the cathode contact can be reduced, resulting in an advantageous aspect that the thickness of the housing over the cathode contact may be 2 through 5 mm.